In the PCM technique, it is known to transform analogue signals that have to be transmitted to an input of a channel, for example a telephone channel, into pulse series or words, each comprising a fixed number of bits. A fixed consecutive number of said words together forms a subframe; a fixed consecutive number of subframes together forms a frame, and a mixed consecutive number of frames will be designated as a multiframe.
Since the bits of a number of channels are combined in time spaces at the transmitting end, as a consequence the combined channels have to be separated at the receiving end. Thus the problem arises of how to make these two processes take place synchronously. In order to let the receiving end know if there is synchronism with respect to the transmitting end, synchronizing signals can be transmitted in significant positions in each word, subframe, frame and/or multiframe. At the receiving end, it can be deduced from the position of the synchronizing signals in the received signal and from the rhythm of the clock located at that end if there is synchronism between the transmitting end and the receiving end. If it appears that this is no longer the case, this synchronism has to be recovered at the receiving end. However, the received signal may also comprise some bit errors, caused by interference pulses or the like. So when at the receiving end a synchronizing signal is not received correctly or in the right position in the signal, there need not always have been a loss of synchronism, since these faults may also have been caused by bit errors. Hence in many of the known systems the loss of synchronism is only ascertained after several synchronizing signals have been received in error. After this the processing of the received signal can be stopped and a search action be started. Then when the expected synchronizing signal is found in the signal, the synchronism can be recovered with the help of the position of said synchronizing signal in the signal. This occurs only after the synchronizing signal has been correctly received several times. So in its simplest form the resynchronizing process comprises a search action and a recovery action.
According to the known technique a resynchronizing device can comprise an "incorrect"-counter and a "correct"-counter, a search action being started when the "incorrect"-counter has reached a certain state, and a recovery action being started when the "correct"-counter has reached a certain state. Such synchronizing methods and devices are much used.
A drawback of the synchronizing method described is that the time between the erroneous reception of a synchronizing signal and the start of the recovery action is relatively long. Moreover, the system becomes more sensitive to bit errors in case the recovery time set is short.